In the 1960s the volume of vehicle transportation by ship started to expand to the extent that a special ship type was developed for this purpose, the basic concept whereof being still in use. In the beginning, it was for the most part passenger cars and vans that were transported on these ships (PCC--Pure Car Carrier type), on an average, the number thereof being several thousand vehicles (about 2000-4000) at a time. The ships returned empty. In the past few years a multipurpose ship type (PCTC--Pure Car & Truck Carrier with a payload of 4000 . . . &gt;6500 passenger cars) has been gaining ground and in which about 20% of the deck area has been dimensioned to receive heavier wheeled or general cargo. When the heavy-load decks are filled with heavy cargo, the cargo carrying capacity of the remaining light decks is decreased significantly. The free space between heavy-load decks is considerably higher than that of normal car decks.
These special ships usually have 10 to 12 cargo decks, and two of these are mainly reserved for transportation of the above mentioned heavier cargo. The heavy-load decks have to be placed relatively high on the level of the deck above the machinery space if it is located in the afterbody, and thus relatively high, which is not a good solution as regards the stability of the ship.
On the heavy-load decks or on some parts thereof containers may also be placed which have to be brought aboard the ship either on wheeled pallets, in which case the pallets remain on the ship, or by special trucks. The containers are placed in stacks of 1 to 2 layers on the decks.
For functional loading and unloading, space is required for drive lanes, openings in transverse bulkheads, sides and decks. The ship has to be equipped with a heavy stern ramp, stern gates, and in general with 1 to 2 side ports. The transverse bulkheads must be provided with openings, and they have to be specially reinforced and equipped with remote controlled actuators. The cargo decks must have openings and be equipped with hoistable drive-lane ramps, of which some are fixed, some hinged or hoistable. In most cases there are also a few lift platforms of articulated type for handling cargo between two decks. The highest decks can be divided by means of hoistable car decks. There are also car decks which are hinged to the side bulkheads and which can be turned by means of actuators into the operating position. All in all, the structures must have a great number of openings and they must be reinforced, there is a lot of bulky equipment, fixed or moveable, in these areas, and space has to be reserved for drive lanes. There are generally 2 to 3 longitudinal pillar rows on the decks, to reduce the hull weight, but at the same time to create restrictions as to the positioning of vehicles and cargo.
The vehicles are driven within the ship using their own engine power. Because of exhaust gases the ventilation system of the ship must be exceptionally effective. A large number of ventilation ducts also splits the deck areas.
The total weight of vehicle carrying ships is also relatively heavy. The vehicles themselves are homogeneous, light transport goods, the stowage factor being on an average four to five times higher compared with container and general cargo. In a pure car carrier the weight of car cargo represents about 40 to 50% of the dead weight of the ship, while in PCTC-type ships it is only about 20 to 25% of the dead weight. In all circumstances, a considerable quantity of so called ballast water has to be transported to ensure the stability of the ship, in the most unfavourable cases the amount thereof exceeding the weight of the vehicle cargo. As a result, more engine power is needed, unnecessary fuel is consumed; besides, the shipping company does not gain anything from transporting "dead water ballast". The deck houses are located on the uppermost deck, and so are the life-boat stations.
The vertical center of gravity of the ship structure being high has been a limiting factor in utilizing the space vertically. In conventional techniques the construction design in the cargo spaces is based on steel plate deck reinforced with stiffening girders. The total thickness of such a local construction may be 200 . . . &gt;450 mm and the plate thicknesses of fixed light-weight car decks are 5 to 6 mm at the minimum, exceeding considerably the local-strength thickness required by the cargo. In a plate field of a deck there are lower beams in each frame space and high frame girders at sparser intervals. On the edges of deck openings and drive ramps there are high, strong stiffening beams. Hoistable or turnable platforms are of lighter construction, shipyard specific, and constructed in accordance with generally known concepts. Said structures also require space either in the roof or on the walls; in addition, actuators need space.
Vehicle transport logistics is going through changes worldwide. Major producers have established and keep on establishing factories in their main export countries, to be in close proximity to end-users. The seasonal character of transports is growing and vehicle transport volumes are decreasing. Car parts and components are transported in increasing quantities. The freer market places demands on greater flexibility in handling different bulk or general cargo, better suitability for handling port and customer-specific small batches etc. on the ships of tomorrow. Economical use of ships calls for a better transport efficiency also during the return voyage. This is often a problem in current ship types. Loading and unloading no longer takes place in only two ports; on the contrary, a ship may have to make 5 to 10 port calls. The current ship types also have weaknesses in loading flexibility. Placing different kinds of customer-specific batches of different sizes on a number of fixed decks and partly on hoistable decks or drive ramps prolongs the loading phase and does not always succeed satisfactorily. The control of batches to be unloaded at a particular port may also lead to new intermediate loadings there. These problems are hard to eliminate using the current basic concept. Such ship types exert global sea traffic on all sea routes.
RO--RO ships have also been developed to handle multicargoes, whereby they are enabled to transport different vehicles as a part of the cargo. In these ship types the cargo is transferred aboard by means of waggon and carriage pallets, which are carried along with the cargo to the port of destination. This method is applied particularly to transporting forest products. To increase loading flexibility, containers are also loaded on these pallets. Straddle carriers and trucks are also used for container handling. A high cargo space can be divided vertically in two or three sections by means of so-called hoistable car decks. The loading and unloading capacity of the ship is satisfactory. All in all, this method is, however, expensive on account of terminal facilities and special ship equipment. Space utilization and stowage efficiency are not good. To facilitate firm fastening of wheeled cargo, the fixed structures of a ship have to be appropriately constructed; separate fastening equipment and plenty of manual work aboard are also needed. The basic decks of the ships are dimensioned for shaft and wheel loads of heavy wheeled cargo, whereby the local strength of the decks is on an average 8 to 20 times higher than is required by a load of passenger cars and vans.
Refrigerated ships form the third significant ship group carrying vehicle cargo, but only as return cargo. In the refrigerated ships cargo is placed on cargo decks in accordance with conventional technique. The cargo is hoisted onto the decks through hatches.
According to U.S. Pat. No. 1,815,687, cars are transported in a cargo ship provided with fixed or adjustable cargo decks. The cars are transferred onto the decks along ramps.
The patent GB 2 406 105 describes a bulk-cargo ship that is convertible into a car carrier. The ship is equipped with a set of adjustable tween decks; the decks are joined together with ramp units. Cars are driven along a ramp between the quay and the ship aboard the ship and into a parking space on an appropriate deck.
Swedish patent SE 345 632 describes a ship carrying car or general cargo on container-dimensioned pallets with support pillars at the corners. The pallets are hoisted from above into wells on the ship just as is done with containers. Support pillars are arranged to support the pallet thereabove. As car lengths vary considerably, cars have to be placed on unnecessarily long pallets of a standard container's length also in this case.
Swedish patent application SE 8304984-1 describes a cargo ship with movable frame structures mounted on the uppermost deck and with deck pontoon elements related thereto. Cars are moved from deck to deck by means of movable ramp--bridge structures located between deck elements.
U.S. Pat. No. 4,106,640 describes a method of transferring cars into a ship by using complicated, winding conveyor elements, in which method the car wheels are put directly onto the conveyor and the cars are transferred onto normal cargo decks.
As has been already described in part, a cargo deck known in the art comprises a plate field and beams thereunder. In all ship types described above, a majority of the cargo decks have been designed, in addition to serve local loads, to carry loadings required by the total strength of the ship. Normally, the thicknesses of the deck plates, in light-weight decks, are at least 5 to 6 mm. The deck plate thickness for heavier shaft loads is 15 to 16 mm. If only the requirements set by the local strength and the loading demands required by conventional cargo were emphasized, a significantly less heavy and less high structure would be sufficient. The total thickness of the deck structures known in the art is of the order of magnitude 200 . . . &gt;450 mm.
In U.S. Pat. No. 3,363,597, a hull structure of a ship is described which comprises a bottom, the sides and a strength deck. The structural parts constitute a uniform shell structure mainly bearing the forces directed at the ship. Thus, the self-supporting shell constitutes the bearing parts of the ship. A space grillage structure has been positioned within the inner parts of the ship, said structure being mounted, for instance, by welding on said bearing shell structure, and in the cells of said space grillage the actual cargo space units or modules are positioned, being uniform space units. Thus, the question is of how to apply a generally known modular structure in a ship. The design described therein is not any more appropriate for the transportation tasks dealt with above than are the rest of the prior art structures as they result in a conventional cargo ship as regards the cargo space arrangements. The design described therein is not at all appropriate for large-scale transportation of cars etc., or at least the payload efficiency is extremely poor.